We have identified major paradigm shifts relative to near-IR filamentation when high power multiple terawatt laser pulses are propagated at mid-IR and long-IR wavelengths within key atmospheric transmission windows. Individual filaments at near-IR (800 nm) wavelengths typically persist only over tens of centimeters, despite the whole beam supporting them being sustained over about a Rayleigh range. In the important mid-IR atmospheric window (3.2 - 4 μm) optical carrier wave self-steepening (carrier shocks) tend to dominate and modify the onset of long range filaments. These shocks generate bursts of higher harmonic dispersive waves that constrain the intensity growth of the filament to well below the traditional ionization limit, making long range low loss propagation possible. For long wavelength pulses in the 8-12 μm atmospheric transmission window, many-electron dephasing collisions from separate gas species act to dynamically suppress the traditional Kerr self-focusing lens and leads to a new type of whole beam self-trapping over multiple Rayleigh ranges. This prediction is key, since strong linear diffraction at these wavelengths are the major limitation and normally requires large launch beam apertures. We will present simulation results that predict multiple Rayleigh range propagation paths for whole beam self-trapping and will also discuss some recent efforts to extend the HITRAN linear atmospheric transmission/refractive index database to include nonlinear responses of important atmospheric molecular constituents.

The influence of microscopic non-equilibrium dynamics on vertical external-cavity surface-emitting lasers (VECSELs) is investigated through a systematic numerical study of single- and dual-wavelength operation. In single-wavelength operation the microscopic dynamics can be adiabatically eliminated, however in dual-wavelength operation the microscopic dynamics varies with the spectral location of the modes. The optically active quantum wells (QWs) are modeled microscopically using the Semiconductor Bloch equations while the CW laser field is simulated using Maxwell’s equations. Higher order correlation terms, such as carrier scattering and polarization dephasing, are treated on the level of second Born-Markov or as effective rates. Results are presented on the modeling, stability, and non-equilibrium effects in dual-wavelength operation.

Here we present the gain and SESAM structure design strategy employed for the demonstration of ultrashort pulses and we present a comprehensive study outlining the influence of the cavity geometry on the pulse duration and peak power achievable with a state of the art VECSEL and SESAM structure. We will discuss the physical mechanisms limiting the output power with near 100fs pulses and we will compare experimental results obtained with different cavity geometries, including a V-shaped cavity, a multi-fold cavity, and a ring cavity in a colliding pulse modelocking scheme. The experimental results are supported by numerical simulations.

Mode-locked VECSELs using SESAMs are a relatively less complex and cost-effective alternative to state-of-the-art ultrafast lasers based on solid-state or fiber lasers. VECSELs have seen considerable progress in device performance in terms of pulse width and peak power in the recent years. However, it appears that the combination of high power and short pulses can cause some irreversible damage to the SESAM. The degradation mechanism, which can lead to a reduction of the VECSEL output power over time, is not fully understood and deserves to be investigated and alleviated in order to achieve stable mode-locking over long periods of time. It is particularly important for VECSEL systems meant to be commercialized, needing long term operation with a long product lifetime.

Here, we investigate the performance and robustness of a SESAM-modelocked VECSEL system under intense pulse intensity excitation. The effect of the degradation on the VECSEL performance is investigated using the SESAM in a VECSEL cavity supporting ultrashort pulses, while the degradation mechanism was investigated by exciting the SESAMs with an external femtosecond laser source. The decay of the photoluminescence (PL) and reflectivity under high excitation was monitored and the damaged samples were further analyzed using a thorough Transmission Electron Microscopy (TEM) analysis. It is found that the major contribution to the degradation is the field intensity and that the compositional damage is confined to the DBR region of the SESAM.

We present preliminary results showing the potential of VECSEL technology for the generation of high power coherent supercontinuum. Among these results, we demonstrate a stable output power of 16 W with a pulse duration of 71 fs and a repetition rate of 1.7 GHz from a VECSEL oscillator and Ytterbium fiber amplifier. This system was used to generate a coherent supercontinuum averaging 3 W of power using a highly nonlinear photonic crystal fiber. In addition, we discuss the possible methods for the detection and stabilization of the carrier offset frequency. The beatnote between a VECSEL seeded supercontinuum and an external CW laser reveals a relatively stable signal, well above the detection noise. A discussion about system design considerations for noise reduction and increased offset frequency stability is also included.

There is a strong push worldwide to develop multi-Joule femtosecond duration laser pulses at wavelengths around 3.5-4 and 9-11μm within important atmospheric transmission windows. We have shown that pulses with a 4 μm central wavelength are capable of delivering multi-TW powers at km range. This is in stark contrast to pulses at near-IR wavelengths which break up into hundreds of filaments with each carrying around 5 GW of power per filament over meter distances. We will show that nonlinear envelope propagators fail to capture the true physics. Instead a new optical carrier shock singularity emerges that can act to limit peak intensities below the ionization threshold leading to low loss long range propagation. At LWIR wavelengths many-body correlations of weakly-ionized electrons further suppress the Kerr focusing nonlinearity around 10μm and enable whole beam self-trapping without filaments.

Since the invention of VECSELs, vast spectral coverage has been demonstrated with emission wavelengths in the range from the UV to almost the MIR. Accordingly, a great variety of different quantum well and quantum dot gain designs have been applied so far to achieve such versatility. A novel gain design for GaAs based VECSELs emitting at wavelengths >1.2 μm employs type-II quantum wells, which exhibit spatially indirect charge-carrier recombination. The first VECSEL based on such a design has been demonstrated very recently. Our device consists of ten (GaIn)As/Ga(AsSb)/(GaIn)As heterostructures arranged as a resonant periodic gain. We summarize the development of this pioneering structure and discuss the fundamental laser characteristics, such as carrier densities, gain temperatures and slope efficiency. Remarkable output powers up to 4 W are demonstrated in multi-transverse mode operation at 1.2 μm. Also, the performance in TEM00 operation is investigated, with an M2 < 1.13. One major difference to conventional type-I gain structures is a characteristic blue shift of the material gain. Due to the importance of the detuning in quantum well based surface-emitters, the blue shift has to be considered as a critical designing parameter. Hence, we carry out a detuning study in order to determine an optimal detuning. As an important part of the optimization, the experimental results are compared with fully microscopic simulations.

We present a passive and robust mode-locking scheme for a Vertical External Cavity Surface Emitting Laser (VECSEL).We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide a stable colliding pulse operation. With this cavity geometry, the two counter propagating pulses synchronize on the SESAM to saturate the absorber together. This minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counter-propagating pulses in the SESAM is shown to extend the range of the modelocking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195fs with an average power of 225mW per output beam at a repetition rate of 2.2GHz, giving a peak power of 460W per beam. The remarkable robustness of the modelocking regime is discussed and a rigorous pulse characterization is presented.

A systematic study of microscopic many-body dynamics is used to analyze a strategy for how to generate ultrashort mode locked pulses in the vertical external-cavity surface-emitting lasers with a saturable absorber mirror. The field propagation is simulated using Maxwell’s equations and is coupled to the polarization from the quantum wells using the semiconductor Bloch equations. Simulations on the level of second Born-Markov are used to fit coefficients for microscopic higher order correlation effects such as dephasing of the polarization, carrier-carrier scattering and carrier relaxation. We numerically examine recent published experimental results on mode locked pulses, as well as the self phase modulation in the gain chip and SESAM.

We present a novel Vertical External Cavity Surface Emitting Laser (VECSEL) cavity design which makes use of multiple interactions with the gain region under different angles of incidence in a single round trip. This design allows for optimization of the net, round-trip Group Delay Dispersion (GDD) by shifting the GDD of the gain via cavity fold angle while still maintaining the high gain of resonant structures. The effectiveness of this scheme is demonstrated with femtosecond-regime pulses from a resonant structure and record pulse energies for the VECSEL gain medium. In addition, we show that the interference pattern of the intracavity mode within the active region, resulting from the double-angle multifold, is advantageous for operating the laser in CW on multiple wavelengths simultaneously. Power, noise, and mode competition characterization is presented.

Since the invention of VECSELs, their great spectral coverage has been demonstrated and emission wavelengths in the range from UV to almost MIR have been achieved. However, in the infrared regime the laser performance is affected by Auger losses which become significant at large quantum defects. In order to reduce the Auger losses and to develop more efficient devices in the IR, type-II aligned QWs have been suggested as alternative gain medium for semiconductor lasers.

Recent development of high power femtosecond pulse modelocked VECSEL with gigahertz pulse repetition rates sparked an increased interest from the scientific community due to the broad field of applications for such sources, such as frequency metrology, high-speed optical communication systems or high-resolution optical sampling. To the best of our knowledge, we report for the first time a colliding pulse modelocked VECSEL, where the VECSEL gain medium and a semiconductor saturable absorber (SESAM) are placed inside a ring cavity. This cavity geometry provides both a practical and an efficient way to get optimum performance from a modelocked laser system. The two counter propagating pulses in our ring cavity synchronize in the SESAM because the minimum energy is lost when they saturate the absorber together. This stronger saturation of the absorber increases the stability of the modelocking and reduces the overall losses of the laser for a given intra-cavity fluence, leading to a lower modelocking threshold. This also allows the generation of fundamental modelocking at a relatively low repetition rate (<GHz) with a higher output power compared to conventional VECSEL cavity. We obtained a total output power of 2.2W with an excellent beam quality, a pulse repetition rate of 1GHz and a pulse duration ranging from 1ps to 3ps. The emitted spectrum was centered at 1007nm with a FWHM of 3.1nm, suggesting that shorter pulses can be obtained with adequate dispersion compensation. The laser characteristics such as the pulse duration and stability are studied in detail.

We demonstrate a low thermal impedance hybrid mirror VECSEL. We used only 14 pairs of AlGaAs/AlAs, transparent at the pump wavelength, and we used a patterned mask to deposit pure gold on areas of the chip to be pumped, and Ti/Au on other area to circumvent the poor adhesion of gold on GaAs. A higher gain is observed on an area metallized with pure gold and an output power of 4W was obtained, showing the effectiveness of the metallic mirror and validating the bonding quality. Chip processing and laser characteristics are studied in detail and compared to simulations.

We present a comprehensive characterization of semiconductor gain and absorber devices utilizing novel measurement techniques. Using a 20fs probe laser, a time resolution in the few femtosecond range is achieved in traditional pump and probe measurements performed on VECSELs and SESAMs. In-situ characterizations of VECSEL samples mode-locked in the sub-500fs regime reveal the fast and longtime recoveries of the gain present in real lasing conditions. Spectrally-resolved probing gives further information about the properties of carriers in VECSEL gain media. Our results indicate that stable mode-locked operation is sustained by multiple carrier relaxation mechanisms ranging from a few femtoseconds to the pico- and nanosecond regimes.

Microscopic many-body theory is employed to analyze the mode-locking dynamics of a vertical external-cavity surface-emitting laser with a saturable absorber mirror. The quantum-wells are treated microscopically through the semiconductor Bloch equations and the light field using Maxwell's equations. Higher order correlation effects such as polarization dephasing and carrier relaxation at the second Born level are included and also approximated using effective rates fitted to second-Born-Markov evaluations. The theory is evaluated numerically for vertical external cavity surface emitting lasers with resonant periodic gain media. For given gain, the influence of the loss conditions on the very-short pulse generation in the range above 100 fs is analyzed. Optimized operational parameters are identified. Additionally, the fully microscopic theory at the second Born level is used to carrier out a pump-probe study of the carrier recovery in individual critical components of the VECSEL cavity such as the VECSEL chip itself and semiconductor or graphene saturable absorber mirrors.

Fully microscopic many-body models are used to determine important material characteristics of GaAsBi and InAsBi
based devices. Calculations based on the band anti-crossing (BAC) model are compared to first principle density
functional theory (DFT) results. Good agreement between BAC-based results and experimental data is found for
properties that are dominated by states close to the bandgap, like absorption/gain and photo luminescence. Using the
BAC model for properties that involve states in the energetic region of the BAC defect level, like Auger losses and free
carrier absorption results in a sharp resonance in the dependence of these quantities for Bismuth concentrations for which
the bandgap becomes resonant with the spin-orbit splitting or the BAC-splitting of the light and heavy hole bands. DFT
calculations show that the BAC model strongly over-simplifies the influence of the bismuth atoms on the bandstructure.
Taking into account the more realistic results of DFT calculations should lead to a reduction of the sharp resonance and
lead to enhancements or suppressions for other Bismuth concentrations and spectral regions.

We present a study of various models for the mode-locked pulse dynamics in a vertical external-cavity surface emitting laser with a saturable absorber. The semiconductor Bloch equations are used to model microscopically the light-matter interaction and the carrier dynamics. Maxwell’s equations describe the pulse propagation. Scattering contributions due to higher order correlation effects are approximated using effective rates that are found from a comparison to solving the microscopic scattering equations on the second Born-Markov level. It is shown that the simulations result in the same mode-locked final state whether the system is initialized with a test pulse close to the final mode-locked pulse or the full field build-up from statistical noise is considered. The influence of the cavity design is studied. The longest pulses are found for a standard V-cavity while a linear cavity and a V-cavity with an high reflectivity mirror in the middle are shown to produce similar, much shorter pulses.

While Vertical-External-Cavity-Surface-Emitting-Lasers (VECSELs) have been successfully used as ultrafast laser sources with pulse durations in the hundreds of femtosecond regime, the dynamics within the semiconductor gain structure are not yet completely understood. With the high carrier densities inside the semiconductor, nonequilibrium effects such as kinetic-hole burning are expected to play a major role in pulse formation dynamics. Moreover, the nonlinear phase change by the intense light field can induce a complex dispersion, which may potentially limit the achievable pulse durations. To shed light on such nonequilibrium dynamics, we perform in-situ characterization of mode-locked VECSELs. We probe the gain media as well as the intracavity absorber with a femtosecond fiber laser source. For measuring temporal characteristics, we employ an asynchronous optical sampling technique by phase-locking the repetition rate of the VECSEL to a multiple of the probe laser with an adjustable offset frequency. This allows for probing dynamics from femtosecond to nanosecond time scales with scan rates up to hundreds of Hertz without compromise of measurement precision which can be introduced by mechanical delays covering such large temporal windows. With a resolution in the femtosecond range, we characterize gain depletion by the intracavity pulse as well as the gain recovery timescales for different power levels and operation regimes.

The influence of non-equilibrium carrier dynamics on pulse propagation through inverted semiconductor gain media is investigated. For this purpose, a fully microscopic many-body model is coupled to a Maxwell solver, allowing for a self-consistent investigation of the light–matter-coupling and carrier dynamics, the optical response of the laser and absorber in the multiple-quantum-well medium, and the modification of the light field through the resulting optical polarization. The influence of the intra-pulse dynamics on the magnitude and spectral dependence of pulse amplification for single pulses passing through inverted quantum-well media is identified. In this connection, the pulse-induced non-equilibrium deviations of the carrier distributions, the kinetic-hole filling kinetics in the gain medium, and the saturable-absorber-relaxation dynamics are scrutinized. While pulses shorter than about 100 fs are found to be rather unaffected by the carrier-relaxation dynamics, the pump-related dynamics become prominent for pulses in the multi-picosecond range leading to significant amplification.

The mode-locking dynamics of a vertical external-cavity surface-emitting laser with saturable absorber is analyzed using a microscopic many-body theory. The light field is treated at the level of Maxwell’s equations and the quantum-wells are modeled using the semiconductor Bloch equations. The carrier relaxation and the polarization dephasing dynamics is treated at different levels of approximation ranging from a simple rate approximation to the second-Born–Markov level. Examples of mode-locked pulse generation are presented identifying the regimes for stable ultra-short pulses, multiple pulse generation, and instability.

After a brief historical review, we describe recent research in the study of tera-Watt class femtosecond lasers propagating in air and condensed media. Here critical self-focusing of the light field reflects the presence of a famous singularity (blow-up in finite time) in the governing Nonlinear Schrö dinger equation (NLS) — this contribution deals with moving into a regime where NLSE fails and more exact optical carrier resolved pulse propagators need to be developed and secondly, addresses the failure of well-established phenomenological nonlinear optical susceptibilities and their replacement by more fundamental quantum models.

We systematically study the single- and multi-mode emission of vertical-external-cavity surface-emitting lasers (VECSELs) using streak camera measurements and interferometric measurement techniques. In all experiments, the VECSEL chip is based on (GaIn)As multi-quantum wells as active medium designed for laser emission around 1010 nm. The emission is analyzed in dependence of the pump power, employing two resonator designs as well as different output couplers. We monitor the evolution of emission bandwidth and show that in our setups a stable two-color lasing –with both lasing intensities sharing the same gain region on the chip– is related to a sufficiently high number of longitudinal modes participating in the laser emission.

We demonstrate a continuous wave, single frequency terahertz (THz) source based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency VECSELs with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and are used as pump sources for the nonlinear down conversion. The emitting THz radiation is centered at 1.9 THz and has a linewidth of less than 100 kHz. The output power of the source exceeds 100 μW. We show that the THz source can be used as local oscillator to drive a receiver used in astronomy applications.

This tutorial gives an overview of the microscopic approach developed to describe equilibrium and nonequilibrium effects in optically excited semiconductor systems with an emphasis to the application for VECSEL modelling. It is outlined how nonequilibrium quantum theory is used to derive dynamic equations for the relevant physical quantities, i.e. the optically induced polarization and the dynamical carrier occupation probabilities. Due to the Coulombic many-body interactions, polarization and populations couple to expectation values of higher-order quantum correlations. With the help of a systematic correlation expansion and truncation approach, we arrive at a closed set of equations. Formally these can be combined with Maxwell’s equations for the classical light field, yielding the Maxwell-semiconductor Bloch equations (MSBE). However, instead of the more traditional approach where losses and dissipative processes are treated phenomenologically and/or through coupling to external reservoirs, we derive fully microscopic equations for the carrier-carrier and carrier-phonon scattering as well as the effective polarization dephasing. Due to their general nature, the resulting equations are fully valid under most experimentally relevant conditions. The theory is applied to model the high-intensity light field in the VECSEL cavity coupled to the dynamics of the optical polarization and the nonequilibrium carrier distributions in the quantum-well gain medium.

We demonstrate the highest free running single frequency power from a single chip VECSEL reported to date, with more than 15W in continuous operation at room temperature. The GaAs-based structure presents an emission wavelength of 1020nm and a tuning range <15nm, with a continuous tunability of 9GHz. The TEM00 output beam exhibits very low transverse phase fluctuations across the entire mode, leading to a beam quality M2 <1.2. To identify and reduce the different sources of noise, the relative intensity noise and frequency noise spectral density are investigated and the intensity and the frequency of the laser were independently stabilized. The laser frequency is controlled and tuned varying the cavity length using a high bandwidth piezoelectric element while intensity fluctuations are reduced by varying the pump intensity. Intrinsic and stabilized frequency and intensity noise are compared.

Fully microscopic models for the calculation of the carrier dynamics and resulting optical response are used to investigate the validity of various models that have been suggested as the cause for the efficiency droop in GaN-based devices. Models based on internal piezoelectric electric fields, carrier localization, Auger and density-activated defect recombination are analysed. In particular, the models are used to simulate aspects of a recent experiment in which green emitting quantum wells were pumped resonantly and emission from adjacent ultra-violet emitting wells was attributed to carrier redistributions due to Auger processes. It is shown that the UV emission can be explained as a direct result of the optical excitation without involving Auger processes.

We report on our research in power scaling VECSEL around 1 μm to exceed 100W per chip. Recently, we have
utilized these optimized VECSEL chips to achieve a new record for a mode-locked VECSEL. The output power
of the laser was 3.4W. This corresponds to a pulse energy of 7.5nJ and a pulse peak power of 13.3kW. Both are
record values for a semiconductor laser in the femtosecond regime. These optimized structures have also been
used to demonstrate high power operation with a highly coherent TEM00 mode and to demonstrate a record
single frequency output power of 15W.

Fully microscopic many-body models are used to calculate the radiative losses in GaN-based light emitting devices. It is shown how simpler models under-estimate these losses significantly. Using the high accuracy of the models allows to eliminate the corresponding loss parameter (B) and its density- and temperature dependence from the space of parameters that are used to fit efficiency data. This allows to study the dependencies of the remaining processes with high accuracy. Using this model, it is show that many processes that have been proposed as causes for the efficiency droop either have wrong dependencies, magnitudes or require unreasonable assumptions to explain the phenomena in general. The most plausible droop model appears to be a combination of carrier delocalization at very low temperatures and pump powers, density- activated defect-recombination at low to medium pumping and injection/escape losses at strong pumping.

The antimonide based vertical external cavity surface emitting lasers (VECSELs) operating in the 1.8 to 2.8 Tm wavelength range are typically based on InGaAsSb/AlGaAsSb quantum wells on AlAsSb/GaSb distributed Bragg reflectors (DBRs) grown lattice-matched on GaSb substrates. The ability to grow such antimonide VECSEL structures on GaAs substrates can take advantage of the superior AlAs based etch-stop layers and mature DBR technology based on GaAs substrates. The growth of such III-Sb VECSELs on GaAs substrates is non-trivial due to the 7.78% lattice mismatch between the antimonide based active region and the GaAs/AlGaAs DBR. The challenge is therefore to reduce the threading dislocation density in the active region without a very thick metamorphic buffer and this is achieved by inducing 90 ° interfacial mist dislocation arrays between the GaSb and GaAs layers. In this presentation we make use of cross section transmission electron microscopy to analyze a variety of approaches to designing and growing III-Sb VECSELs on GaAs substrates to achieve a low threading dislocation density. We shall demonstrate the failure mechanisms in such growths and we analyze the extent to which the threading dislocations are able to permeate a thick active region. Finally, we present growth strategies and supporting results showing low-defect density III-Sb VECSEL active regions on GaAs.

The broad gain-bandwidth and the high output powers make vertical-external-cavity surface-emitting lasers (VECSELs) promising candidates as femtosecond laser sources. Besides an effective design of the gain structure, the major challenges for high power VECSELs are the thermal management of the chips as well as the homogeneity of the epitaxial growth. In this work, we present results of passively mode-locking of our highly efficient VECSELs and demonstrate femtosecond operation with record output powers and pulse energies. At a repetition rate of 1.7 GHz, we achieved nearly transform-limited pulses with 682 fs duration and an average output power exceeding 5 W.

We demonstrate the utility of optically pumped semiconductor lasers (OPSLs) in the eld of precision atomic spectroscopy. We have constructed an OPSL for the purpose of laser-cooling and trapping neutral Hg atoms. The OPSL lases at 1015 nm and is frequency quadrupled to provide the trapping light for the ground state cooling transition. We report up to 1.5 W of stable, single-frequency output power with a linewidth of < 70 kHz with active feedback. From the OPSL we generate deep-UV light at 253.7 nm used to form a neutral Hg magneto-optical trap (MOT). We present details of the MOT. We also report initial results for spectroscopy of the 61S0 - 63P0 clock transition in the Hg199 isotope.

We report on our research in power scaling OPSL around 1 μm to exceed 100W per chip by combining a rigorous quantum design of an optimized MQW epitaxial structure, highly accurate and reproducible wafer growth and an efficient thermal management strategy. Recently we have utilized these state-of-the-art optimized OPSL chips to achieve a new record for a mode-locked OPSL with an intra-cavity SESAM. The average output power of the laser in the optimum operation point of mode-locked operation was 5.1W while being pumped with 25W of net pump power. This corresponds to a pulse energy of 3 nJ and a pulse peak power of 3.8 kW.

We present a terahertz source based on difference frequency generation within a laser cavity. Combining the high
intracavity intensities of a dual-color vertical external cavity surface emitting laser (VECSEL) with the high nonlinear
coefficient of a periodically poled lithium niobate crystal enables the generation of milliwatts of continuous wave
terahertz radiation. As the frequency spacing between the two simultaneously oscillating laser lines can be adjusted
freely, the entire range of the terahertz gap can be covered. We discuss different approaches for the wavelength control
of the dual-color laser sources as well as emission characteristics of the nonlinear crystal. Exemplarily, we chose the
frequencies 1.9 THz to characterize the source in term of the beam shape, the linewidth and power scalability. To
investigate the emitted THz spectrum, heterodyne detection is employed.

We investigate experimentally and theoretically the influence of non-radiative carrier losses on the performance of
VECSELs under pulsed and CW pumping conditions. These losses are detrimental to the VECSEL performance
not only because they reduce the pump-power to output-power conversion efficiency and lead to increased
thresholds, but also because they are strong sources of heat. This heating reduces the achievable output power
and eventually leads to shut-off due to thermal roll-over. We investigate the two main sources of non-radiative
losses, defect recombination and Auger losses in InGaAs-based VECSELs for the 1010nm-1040nm range as well
as for InGaSb-based devices for operation around 2μm. While defect related losses are found to be rather
insignificant in InGaAs-based devices, they can be severe enough to prevent CW operation for the InGaSb-based
structures. Auger losses are shown to be very significant for both wavelengths regimes and it is discussed how
structural modifications can suppress them. For pulsed operation record output powers are demonstrated and
the influence of the pulse duration and shape is studied.

Strategies for power scaling VECSELs, including improving thermal management, increasing the quantum well
gain/micro-cavity detuning that increases the threshold but increases roll-over temperature, and double-passing the
excess pump via reflection from a metalized reflector at the back of a transparent distributed Bragg reflector (DBR) were
studied. The influence of the heat spreader thickness and the pump profile on the temperature rise inside the active
region was investigated using commercial finite element analysis software. Improvement was observed in optical
efficiency of the VECSEL devices with a transparent DBR by double passing the pump light. Higher dissipated power at
maximum output power was found in devices with larger spectral detuning between the quantum well gain and the
micro-cavity detuning.

The microscopic theory for the nonequilibrium optical properties of VECSELs is summarized. Detailed experiments
of VECSELs under two-color operation conditons are performed utilizing streak camera measurements
of the laser output. A statistical analysis reveals the stability range of two-color emission and shows that this
operation mode is possible even in the presence of relatively large losses.

Here we report on the development and demonstration of a tunable high power single frequency Vertical External Cavity
Surface Emitting Laser (VECSEL) operating at 589nm. A highly strained InGaAs/GaAs VECSEL designed to operate at
~ 1178nm is used in conjunction with an intracavity Birefringent Filter (BF) and low finesse Fabry-Perot (FP) etalon to
achieve the single frequency operation at the fundamental wavelength. An internal non-linear optical element is then
used to obtain the single frequency output at the desired wavelength of 589nm. Our results show outputs in excess of
4W at 589nm with a FWHM linewidth of the fundamental frequency to be less than 10MHz. We demonstrate the
measurement of the sodium D1 and D2 lines by passing the output through a reference cell.

In this paper we report on the wavelength tuning of a VECSEL by changing the cavity geometry. The development and
demonstration of a tunable high power single frequency Vertical External Cavity Surface Emitting Lasers (VECSEL)
operating at various wavelengths from the UV to the IR region of the spectrum have been reported in many papers.
However, it is important to understand that in many instances a precise lasing wavelength is required for proper
operation. For example, VECSELs have been designed to specifically interact with the sodium spectral lines. If the
VECSEL growth is not adequate, it may not be possible to reach the desired wavelength in a traditional cavity where the
intracavity mode interacts with the VECSEL chip at normal incidence. Here we notice that if a fold angle is introduced
at the VECSEL chip, a spectral blue shift occurs, and extended tunability may be possible. Therefore, by altering the
cavity geometry it may be possible to further optimize a VECSEL design to obtain maximum output power at a desired
wavelength.

We report on the first observation of intracavity laser cooling inside of a vertical external-cavity surface-emitting laser
(VECSEL). A Yb:YLF crystal is placed under Brewster angle inside the cavity of an InGaAs quantum well VECSEL
emitting around 1030 nm. With the crystal in air, we observed cooling by about 0.5 degrees. By placing the sample and
cavity end mirror inside a vacuum chamber, with the window also at Brewster angle to the laser mode, cooling by 20
degrees has been realized. Furthermore, the development of a compact and efficient integrated cryocooler device is
underway.

An approach based on fully microscopically computed material properties like gain/absorption, radiative
and Auger recombination rates are used to design, analyze and develop optimization strategies for Vertical
External Cavity Surface Emitting Lasers for the IR and mid-IR with high quantitative accuracy. The microscopic
theory is used to determine active regions that are optimized to have minimal carrier losses and
associated heating while maintaining high optical gain. It is shown that in particular for devices in the
mid-IR wavelength range the maximum output power can be improved by more than 100% by making rather
minor changes to the quantum well design. Combining the sophisticated microscopic models with simple onedimensional
macroscopic models for optical modes, heat and carrier diffusion, it is shown how the external
efficiency can be strongly improved using surface coatings that reduce the pump reflection while retaining the
gain enhancing cavity effects at the lasing wavelength. It is shown how incomplete pump absorption can be
reduced using optimized metallization layers. This increases the efficiency, reduces heating and strongly improves
the maximum power. Applying these concepts to VECSELs operating at 1010nm has already resulted
in more than twice as high external efficiencies and maximum powers. The theory indicates that significant
further improvements are possible - especially for VECSELs in the mid-IR.

We present an overview of the quantum design, growth and lasing operation of both IR and mid-IR OPSL
structures aimed at extracting multi-Watt powers CW and multi-kW peak power pulsed. Issues related to
power scaling are identified and discussed. The IR OPSLs based on InGaAs QW bottom emitters targeted at
wavelengths between 1015nm and 1040nm are operated in CW mode (yielding a maximum power of 64W)
and pulsed (peak power of 245W). The mid-IR top emitter OPSLs designed to lase at 2μm are based on a
novel lattice mismatched growth using InGaSb QWs and yield a maximum peak power of 350W pulsed.

We demonstrate a novel epitaxial process for the growth of low-dislocation density GaSb on GaAs. The
growth mode involves the formation of large arrays of periodic 90° misfit dislocations at the interface
between the two binary alloys which results in a completely strain relieved III-Sb epi-layer without the
need for thick buffer layers. This epitaxial process is used for the growth of antimonide active regions
directly on GaAs/AlGaAs distributed Bragg Reflectors (DBRs) resulting in 2 μm VECSELs on GaAs
substrates.

Vertical external cavity surface emitting lasers (VECSELs) are attractive for many applications due to their high-power,
high-brightness outputs. In order to power scale the devices, the pump spot size should be increased. However, the large
pump area greatly amplifies the guided spontaneous emission in the epitaxial plane. In order to efficiently power scale
the devices, amplified spontaneous emission (ASE) and lateral lasing must be reduced. We begin by first reporting on
the temperature dependence of the phenomena. Particularly, since the quantum well gain and bandgap are functions of
temperature, ASE and lateral lasing are greatly dependent on the operating temperature as well as the pump power. The
easiest method of quantifying the affect of ASE and lateral lasing is by removing the Fabry-Perot cavity formed by the
chip edges. We have chosen two different methods: Reducing the Fresnel reflections by patterning the edges of the
sample, and depositing a layer of Ge on the edges of the VECSEL chip as the high index of refraction for Ge should
reduce the Fresnel reflections and the absorption properties in the NIR regime should also act to prevent feedback into
the pump area. Our research shows both of these methods have increased the performance and visibly decreased the
amount of lateral lasing seen in the devices.

We compare an InAs quantum dot (QD) vertical external-cavity surface-emitting laser (VECSEL) design consisting of 4
groups of 3 closely spaced QD layers with a resonant periodic gain (RPG) structure, where each of the 12 QD layers is
placed at a separate field antinode. This increased the spacing between the QDs, reducing strain and greatly improving
device performance. For thermal management, the GaAs substrate was thinned and indium bonded to CVD diamond. A
fiber-coupled 808 nm diode laser was used as pump source, a 1% transmission output coupler completed the cavity. CW
output powers over 4.5 W at 1250 nm were achieved.

Vertical external cavity surface emitting lasers (VECSELs) provide a laser design platform in order to explore
a variety of systems, and their flexibility eases this exploration. Moreover, their high-brightness operation
makes them attractive for many applications. In considering the methods of coupling VECSELs as well as
their potential uses, we begin by reporting on the development of a gain coupled VECSEL for use in optical
switching. In particular, two VECSEL cavities share a common gain region; the competition for a common set
of carriers dictate how these cavities interact. The easiest manifestation to realize gain coupling is to utilize
a linear cavity as well as a v-cavity, built around a single half-vertical cavity surface-emitting laser (VCSEL)
chip. The cavity gain/loss of each cavity can be controlled independently through use of birefringent filters,
allowing us to explore the design space, which can be divided up into coarse behavior, easy to analyze through
comparing the two uncoupled lasers, and a fine behavior, where one cavity will affect the other and each cavity
can lase simultaneously, sometimes at dramatically different wavelengths. These two regions may be explained
with simple rate equations, and it will be shown that if prepared properly, spontaneous emission plays a large
role in balancing the two laser cavities within the fine regime, while may be completely neglected in the coarse
regime.

A model based on density-activated defect recombination processes is proposed as a possible explanation
for the efficiency droop in GaN-based lasers. The model yields very good agreement with experimentally
measured efficiencies based on fit parameters that indicate the presence of two types of recombination centers
that have different local distributions and recombination rates. The recombination rates of the two types are
found to be very similar for devices operating at 530nm and 410nm.

Vertical external cavity surface emitting lasers (VECSELs) have captured the interest of high-brightness semiconductor
researchers, primarily due to their simplicity in design, power scalability, and "open cavity architecture,"
wherein it is simple to integrate nonlinear elements into the cavity. Through direct emission and indirect
(frequency-converted) means, wavelengths from the UV through to the mid-wave infrared regimes have been
demonstrated, increasing the suitability of the VECSEL platform for multiple applications. This presentation
outlines recent progress in VECSELs, measurements, novel cavities, and potential applications for these lasers.

The quantum design of VECSEL structures is discusssed using a commercially available design tool. Examples
of realized structures are presented and comparisons between experimental results and modelling predictions are
shown.

Laser beam transformation utilizing the effect of multimode interference in multimode (MM) optical fiber is
thoroughly investigated. When a Gaussian beam is launched to an MM fiber, multiple eigenmodes of the MM fiber are
excited. Due to interference of the excited modes, optical fields that vary with the MM fiber length and the signal
wavelength are generated at the output facet of the MM fiber. Diffractive propagation of these confined fields can yield
various desired intensity profiles in free space. Our calculations show that, an input fundamental Gaussian beam can be
transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and low-divergence Bessel-like
within either the Fresnel or the Fraunhofer diffraction range, or even in both ranges. Experiments on a monothic fiber
beam transformers consisting of a short piece of MM fiber (~ 10 mm long) and a single-mode signal delivery fiber were
carried out. The experimental results indicate the functionality and high versatility of this simple fiber device. The
performance of this fiber device can be easily and widely manipulated through parameters including the ratio between
the core diameters of the SM and MM fiber segments and the length of the MM fiber segment. In addition, the intensity
profile of the output beam can be controlled by tuning the signal wavelength even after the fiber device is fabricated.
Most importantly, this technique is highly compatible with the technology of high power fiber lasers and amplifiers and
fiber delivery systems.

We propose a method of optical data storage that exploits the small dimensions of metallic nano-particles
and/or nano-structures to achieve high storage densities. The resonant behavior of these particles (both individual and in
small clusters) in the presence of ultraviolet, visible, and near-infrared light may be used to retrieve pre-recorded
information by far-field spectroscopic optical detection. In plasmonic data storage, a femtosecond laser pulse is focused
to a diffraction-limited spot over a small region of an optical disk containing metallic nano-structures. The digital
information stored in each bit-cell modifies the spectrum of the femtosecond light pulse, which is subsequently detected
in transmission (or reflection) using an optical spectrum analyzer. We present theoretical as well as preliminary
experimental results that confirm the potential of plasmonic nano-structures for high-density optical storage applications.

Design of optimized semiconductor optically-pumped semiconductor lasers (OPSLs) depends on many ingredients
starting from the quantum wells, barrier and cladding layers all the way through to the resonant-periodic gain (RPG) and
high reflectivity Bragg mirror (DBR) making up the OPSL active mirror. Accurate growth of the individual layers
making up the RPG region is critical if performance degradation due to cavity misalignment is to be avoided.
Optimization of the RPG+DBR structure requires knowledge of the heat generation and heating sinking of the active
mirror. Nonlinear Control Strategies SimuLaseTM software, based on rigorous many-body calculations of the
semiconductor optical response, allows for quantum well and barrier optimization by correlating low intensity
photoluminescence spectra computed for the design, with direct experimentally measured wafer-level edge and surface
PL spectra. Consequently, an OPSL device optimization procedure ideally requires a direct iterative interaction between
designer and grower. In this article, we discuss the application of the many-body microscopic approach to OPSL devices
lasing at 850nm, 1040nm and 2μm. The latter device involves and application of the many-body approach to mid-IR
OPSLs based on antimonide materials. Finally we will present results on based on structural modifications of the
epitaxial structure and/or novel material combinations that offer the potential to extend OPSL technology to new
wavelength ranges.

A frequency-converted optically pumped semiconductor laser (OPSL) is described. The 976-nm OPSL is frequency
doubled intracavity and is forced to operate in single longitudinal mode. An external resonator, containing a cesium
lithium borate crystal is locked to the 488-nm fundamental, generating the second harmonic at 244 nm. Continuous
wave output in excess of 200 mW is generated.

Ultrafast intense femtosecond laser pulses spontaneously undergo critical collapse in air and
condensed media above some critical power. In normally dispersive media, such pulses can
spontaneously generate dynamical X-waves where distinct X-features appear in the spectrally-resolved
far-field. These nonlinear self-trapped pulses resemble linear Bessel beams - the latter
exhibit extended line rather than point foci and are robust to strong perturbations. Nonlinear X-waves
can be directly generated by using an axicon lens and have the potential to generate
highly nonlinear, extended interaction zones relative to pulses with Gaussian profiles. Potential
applications of these pulsed sources to controlling and extending white light supercontinuum and
plasma channel generation will be discussed. X-wave generation in normally dispersive media is
associated witha cascade of pulse splittings where individual split pulses have been identified
with different arms of the spectrally observed X-feature. This allows for novel pump-probe
experiments where a seed pulse can selectively generate Raman Stokes shifted waves by
scattering off of different arms of the X-feature. We will discuss a 3-wave interaction picture that
allows for a transparent physical interpretation of these complex spatio-temporal events.

Optically pumped semiconductor vertical-external-cavity surface-emitting laser (VECSEL) potentially provides an
innovative approach to low-cost frequency agile lasers engineered for specific applications in infrared and visible range.
In this paper, we report on the development and demonstration of a multi-Watt highly strained InGaAs/ GaAs vertical-external-
cavity surface-emitting laser (VECSEL), which can be tuned from 1147 nm to 1197 nm. Based on this tunable
InGaAs/GaAs VECSEL and intracavity frequency doubling, we develop multi-Watt frequency-doubled tunable
VECSEL in a wide yellow-orange band (579 ~595 nm). This compact high-power yellow-orange laser provides an
innovative approach to an affordable guidestar laser (~589.1 nm) solution, and has a lot of important applications in
biomedicine.

Optically pumped semiconductor (OPS) vertical-external-cavity surface-emitting lasers (VECSELs) offer the first truly high-brightness high power laser sources with serious power scaling potential to multiple kW levels and flexible spectral coverage from IR to mid-IR. Due to the fact that the semiconductor chip (or subcavity) of a VECSEL serves as both the gain medium and a cavity mirror, the design and optimization of the semiconductor subcavity is key to achieve high power operation and consequently high power extraction via pump area scaling. A fundamental microscopic quantum design approach, allowing for calculating the electro-optical properties of QWs such as the optical gain/absorption and carrier recombination rates, is combined with a coupled optical-thermal-carrier analysis scheme to design and optimize VECSEL chips for wavelengths in the IR. We will describe the design and optimization procedure and present simulation results on VECSEL chips at wavelengths of 980 nm, 1178 nm, and 2 μm.

Fully microscopic many-body models are used to ivestigate the temperature dependence of radiative and Auger losses in semiconductor lasers. Classical estimates based on simplified models predict carrier density independent temperature dependencies, 1/T for the radiative losses and a temperature activated exponential dependence for the Auger losses. Instead, the micorscopic models reveal for the example of a typical InGaAsP-based structure a 1/T3-dependence for the radiative losses at low carrier densities. For high densities this dependence becomes much weaker and deviates from a simple power law. Auger losses can be described by an exponential dependence for limited temperature ranges if a density dependent activation energy is used. For the threhsold carrier density a temperature dependence close to T2 is found instead of the linear temperature dependence assumed by the simplified models.

Performance metrics of every class of semiconductor amplifier or laser system depend critically on semiconductor QW
optical properties such as photoluminescence (PL), gain and recombination losses (radiative and nonradiative). Current
practice in amplifier or laser design assumes phenomenological parameterized models for these critical optical properties
and has to rely on experimental measurement to extract model fit parameters. In this tutorial, I will present an overview
of a powerful and sophisticated first-principles quantum design approach that allows one to extract these critical optical
properties without relying on prior experimental measurement. It will be shown that an end device L-I characteristic can
be predicted with the only input being intrinsic background losses, extracted from cut-back experiments. We will show
that textbook and literature models of semiconductor amplifiers and lasers are seriously flawed.

The Bloch modes of a periodic slit array in a metallic slab are identified, then used to
investigate the transmission of light through sub-wavelength slits residing in a finite-thickness slab.
Specifically, the Bloch mode method is used here to study Fabry-Perot-like resonances within
individual slits, in conjunction with the onset of surface plasmon polariton (SPP) resonances and in
the vicinity of the Wood anomalies. Although the results largely agree with our earlier numerical
simulations obtained with the Finite-Difference-Time-Domain (FDTD) method, there are
indications that the FDTD method has difficulty with convergence at and around resonances; the
points of agreement and disagreement between the two methods are discussed in the present paper.
When the period p of the slit array is comparable to (or somewhat below) the incident wavelength
λo, the Bloch mode method requires only the 10-20 lowest-order modes of the slit array to achieve
stable solutions; we find the Bloch mode method to be an effective tool for studying dielectric-filled
apertures in highly conductive hosts.

We propose using high power, high brightness optically pumped vertical-external-cavity semiconductor lasers
(VECSELs) as sources to build a partially coherent beam for laser communications applications. VECSELs are compact
wavelength tunable, multi-Watt sources emitting light in a near TEM00 mode. Our theory suggests that the scintillation
index at a remote receiver can be significantly reduced by filling the transmitter aperture with an array of beams. An
experiment will be reported on that confirms the theory predictions and demonstrates further that the reduction in
scintillation index carried through to case of strong turbulence where our perturbation theory fails.

An all-fiber approach is utilized to phase lock and select the in-phase supermode of compact multicore fiber lasers.
Based on the principles of Talbot imaging and waveguide multimode interference, the fundamental supermode is
selectively excited within a completely monolithic fiber device. The all-fiber device is constructed by simply fusion
splicing passive non-core optical fibers of controlled lengths at both ends of a piece of multicore fiber. Experimental
results upon in-house-made 19- and 37-core fibers are demonstrated, which generate output beams with high-brightness
far-field intensity distributions. The whole fabricated multicore fiber laser device can in principle be a single fiber chain
that is only ~10 cm in length, aligning-free in operation, and robust against environmental disturbance.

We study the scintillation index of N partially overlapping lowest order Gaussian laser beams with different wavelengths in weak atmospheric turbulence. Assuming a Von Karman turbulence spectrum and slow detector response and using the Rytov approximation we calculate the longitudinal and radial components of the scintillation
index for typical free-space laser communication setups. We find the initial beam separation that minimizes the longitudinal scintillation and corresponds to the optimal beam configuration. Further reduction of the longitudinal scintillation is obtained by optimizing with respect to both initial beam separation and initial spot size. The
longitudinal scintillation of the optimal N-beam configurations is inversely proportional to N, resulting in a 92% reduction for a 9-beam system compared with the single beam value. The existence of the minimum of longitudinal scintillation is not very sensitive to
the form of the turbulence spectrum. Moreover, the radial scintillation values for the optimal N-beam configurations are found to be significantly smaller than the corresponding single beam values, and this reduction effect also grows with increasing N.

We present an overview of a novel first principles quantum approach to designing and optimizing
semiconductor QW material systems for target wavelengths. Using these microscopic inputs as basic building
blocks we predict the L-I characteristic for a low power InGaPAs ridge laser without having to use adjustable
fit parameters. Finally we employ these microscopic inputs to develop sophisticated simulation capabilities
for designing and optimizing end packaged high power laser structures. As an explicit example of the latter,
we consider the design and experimental demonstration of a vertical external cavity semiconductor laser
(VECSEL).

Two formulations of the Lorentz law of force in classical electrodynamics yield identical results for the total force (and total torque) of radiation on a solid object. The object may be surrounded by the free space or immersed in a transparent dielectric medium such as a liquid. We discuss the relation between these two formulations and extend the proof of their equivalence to the case of solid objects immersed in a transparent medium.

The fundamental physics of high-field laser-matter interactions has driven ultrashort pulse generation to achieve record power densities of 1022 Watts per cm2 in focal spot sizes (FWHM) of 0.8 μm1. These enormous fields are generated by compressing longer, high energy pulses to ever shorter lengths using so-called CPA compressors. Great care has to be taken to achieve such record power densities by controlling the spatio-temporal shape during pulse compression. Despite these remarkable experimental achievements, there have been relatively few developments on the theoretical side to derive realistic physical optical material models coupled to sophisticated E.M propagators. Many of the theoretical analysis tools developed in this emerging field of extreme nonlinear optics are restricted to oversimplified 1D models that completely ignore the complex vector spatio-temporal couplings occurring within such small nonlinear interaction volumes.
The advent of these high power ultra-short pulsed laser systems has opened up a whole new vista of applications and computational challenges. The applications space spans relatively short propagation lengths of centimeters to meters to a target up to many kilometers in atmospheric propagation studies. The high local field intensities generated within the pulse can potentially lead to electromagnetic carrier wave shocking so it becomes necessary to fully resolve the optical carrier wave within the 3D propagating pulse envelope. High local field intensities also lead to an explosive growth of the white-light supercontinuum spectrum and the intensities of even remote spectral components can be high enough to generate nonlinear coupling to the host material. For this reason, spectrally local models of light-matter coupling are expected to fail.
In this paper, we will present a fully carrier-resolved E.M. propagator that allows for few meter long propagation lengths while fully resolving the optical carrier wave. Our applications focus will be on the relatively low intensity regime where critical self-focusing collapse in air or water can lead to very strong non-paraxial ultra-broadband excitations. One reason for this restriction is that we do not yet have computationally feasible robust physical models for ultra-broadband excitation of materials where nonlinear dispersion and absorption become dominant. The propagation of terawatt femtosecond duration pulses in the atmosphere can be qualitatively captured by physical models that include reliable linear dispersion/absorption while treating the nonlinear terms as spectrally local. We will review some recent experimental results by the German-Franco Teramobile team on atmospheric propagation, penetration through obscurants and remote laser induced breakdown spectroscopy. As a second application example will address the issue of strongly non-paraxial spectral superbroadening of femtosecond pulses while propagating in water - these latter nonlinear interactions generate so-called nonlinear X- and O-waves depending on the optical carrier wavelength of the initial pulse.

A dynamical laser model is coupled to a fully microscopic calculation of scattering rates, allowing effcient calculations without phenomenological parameters. The approach is used to analyze nonequilibrium effects in the switch-on of an optically pumped laser structure. Lasing leads to kinetic hole burning in both electron and hole distribution. The gain spectrum, however, does not show spectrally narrow hole burning but a reduction over a wide range of frequencies compared to the equilibrium gain because of the large homogeneous broadening in the high density lasing system.

Various samples from the GaInNAs dilute nitride material system are modeled microscopically and good agreement with experiment is shown for the optical gain, linewidth enhancement factor, photomodulated reflectance and photoluminescence. Even though the differential gain is reduced by the inclusion of nitrogen, the linewidth enhancement factor is shown to stay almost unchanged. Radiative decay times are calculated and show a strong change in their density dependence above threshold.

In this paper we present the development and demonstration of multi-watts highbrightness vertical-external-cavity surface-emitting lasers (VECSELs). Over 10 W TEM00 continuous-wave (CW) output power with high efficiency is demonstrated. Tunable multi-watts VECSELs with over 20 nm tuning range and narrow linewidth are achieved. Potential applications of tunable VECSELs are introduced.

It is demonstrated that fully microscopic many-body models are required for a correct description of the dominant carrier loss processes in semiconductor lasers, spontaneous emission and Auger recombination, and that they are able to quantitatively predict these losses. The density dependence of the losses assumed in semi-empirical approaches, J=AN+BN2+CN3, is shown to break down already near transparency. For the spontaneous emission it is shown to decrease from quadratic to linear (BN), Auger rates are shown to
increase only quadratically (CN2) or even less.

Heavily doped active fibers based on the soft phosphate glass offer an attractive gain medium for compact and high-power laser oscillators. We report a passively modelocked fiber oscillator at 1.5μm based on such active fiber. The standing-wave laser cavity consists of a 20cm-long piece of the side-pumped active phosphate fiber which is heavily co-doped with Er and Yb ions, and a low-ratio fused coupler. The length of the all-fiber laser cavity is 65cm. The modelocked operation of the oscillator is started and sustained by a Semiconductor Saturable Absorber Mirror (SESAM), and no additional pulse narrowing mechanism is used. In order to avoid a premature over-saturation of the SESAM, the fiber end which is butt-coupled to the SESAM is adiabatically tapered which expands the propagating fiber mode and decreases the power density incident on the absorber substantially. The stable modelocked operation of the laser oscillator occurs in the range between 0.65W and 2.3W of the average output power, which is limited by the maximum available pump power at 975nm. The peak pulse power is limited by the saturated SESAM at ~450W, and the pulse width grows from 11psec to 35psec as the pump power is increased. At the pulse repetition rate of 160MHz, the pulse energy reaches 14.4nJ. Our laser oscillator combines the convenience of the all-fiber construction with the power performance previously achievable only with the modelocked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.

Rapid progress in recent years in the development of high power ultrashort pulse laser systems has opened up a whole new vista of applications and computational challenges. Amongst those applications that are most challenging from a computational point of view are those involving explosive critical self-focusing with concomitant explosive growth in the generated light spectrum. Moreover, new experimental developments in the field of extreme nonlinear optics will require more rigorous propagation models beyond those existing in the current literature. Specific applications areas chosen for illustration in this paper include atmospheric light string propagation and nonlinear self-trapping in condensed media. These examples exhibit rather different aspects of intense femtosecond pulse propagation and demonstrate the robustness and flexibility of the unidirectional Maxwell propagator.
A novel aspect of our approach is that the pulse propagator is designed to faithfully capture the light-material interaction over the broad spectral landscape of relevance to the interaction. Moreover the model provides a seamless and physically self-consistent means of deriving the many ultrashort pulse propagation equations presented in the literature.

Semiconductor quantum well active structures are pervasive in many applications of defense related systems ranging from low power edge (DFB), VCSEL and VCSEL emitter arrays to high power low brightness broad area emitters and diode bars. Recent breakthroughs in the development of a new class of high brightness vertical external cavity (VECSEL) emitters offers the potential to replace solid state YAG kW-class laser weapons systems. Remarkably, despite the maturity and dramatic improvement in quality of semiconductor QW growth over the past three decades, there has been no truly predictive means of designing the semiconductor active structure and fast-tracking to a final packaged device. We will describe a fully self-consistent microscopic many-body approach to calculate optical gain, absorption, refractive index spectra and nonradiative recombination rates for a broad class of semiconductor quantum well material systems. The theoretical calculations are free of ad hoc parameter adjustments and provide, for the first time, a means of designing an active semiconductor epi-structure in a predictive manner.

Optically-pumped vertical external cavity semiconductor lasers offer the exciting possibility of designing kW-class solid state lasers that provide significant advantages over their doped YAG, thin-disk YAG and fiber counterparts. The basic VECSEL/OPSL (optically-pumped semiconductor laser) structure consists of a very thin (approximately 6 micron thick) active mirror consisting of a DBR high-reflectivity stack followed by a multiple quantum well resonant periodic (RPG) structure. An external mirror (reflectivity typically between 94%-98%) provides conventional optical feedback to the active semiconductor mirror chip. The "cold" cavity needs to be designed to take into account the semiconductor sub-cavity resonance shift with temperature and, importantly, the more rapid shift of the semiconductor material gain peak with temperature. Thermal management proves critical in optimizing the device for serious power scaling. We will describe a closed-loop procedure that begins with a design of the semiconductor active epi structure. This feeds into the sub-cavity optimization, optical and thermal transport within the active structure and thermal transport though the various heat sinking elements. Novel schemes for power scaling beyond current record performances will be discussed.

Vertical external cavity surface emitting lasers (VECSELs) have been considered the “ultimate disk-laser” due
to their extremely thin active regions and because they take advantage of the high gain found in semiconductor
material. This paper discusses power scaling limitations, including heating effects, surface roughness losses, and
laterally guided amplified spontaneous emission (ASE).

The Finite-Difference Time-Domain (FDTD) method is often a viable alternative to other computational methods used for the design of sub-wavelength components of photonic devices. We describe an FDTD based grid refinement method, which reduces the computational cell size locally, using a collection of nested rectangular grid patches. On each patch, a standard FDTD update of the electromagnetic fields is applied. At the coarse/fine grid interfaces the solution is interpolated, and consistent circulation of the fields is enforced on shared cell edges. Stability and accuracy of the scheme depend critically on the update scheme, space and time interpolation, and a proper implementation of flux conditions at mesh boundaries. Compared to the conformal grid refinement, the method enables better efficiency by using non-conformal grids around the region of interest and by refining both space and time dimensions, which leads to considerable savings in computation time. We discuss the advantages and shortcomings of the method and present its application to the problem of computation of a quality factor of a 3-D photonic crystal microcavity.

We propose the general idea of constructing an ultra-compact optical pickup based on photonic crystals. A few optical components necessary for various functions of an optical head are designed and analyzed.

We demonstrate the integration of microscopic gain calculation into the laser design tool LaserMOD, which is derived from the Minilase II simulator. A microscopic many body theory of the semiconductor allows for the accurate modeling of the spectral characteristics of the material gain. With such a model, the energetic position of the gain peak, the collision broadening, and therefore, the absolute magnitude of the gain can be predicted based solely on material parameters [2]. In contrast, many simpler approaches rely on careful calibration of model parameters requiring additional effort due to fabrication of samples and experimental studies. In our full scale laser simulation multi dimensional carrier transport, interaction with the optical field via stimulated and spontaneous emission, as well as the optical field itself is computed self consistently. We demonstrate our approach on an example of a Fabry-Perot laser structure with GaInAsP multiple quantum wells for 1.55 μm emission wavelength.

The problem of delivering a multimode pump to a single mode core is considered. Original designs of double-clad fiber amplifiers are analyzed. A slab pump geometry of a fiber amplifier is suggested. The partially coherent pump should be coupled to the narrow slab which supports propagation of the pump and absorbs highest modes of the core, then the core itself has no need to be single-mode. The slab plays the role of cladding in conventional double-clad fibers. Such a cladding has no need to surround the core and its refractive index can be higher than that of the core. Then the highest modes of the slab are absorbed at the very beginning of the amplifier. This allows for tapering of the slab, providing almost constant pump power in the core. The end result is a the high efficiency of a compact device with scaling to high pump power. We provide the key design formulars for such a device.

A general scheme for the determination of vital operating characteristics of semiconductor lasers from low intensity photo-luminescence spectra is outlined and demonstrated. A fully microscopic model for the optical properties is coupled to a drift-diffusion model for the mesoscopic charge and field distributions to calculate luminescence and gain spectra in barrier-doped laser material. Analyzing experiments on an optically pumped multi quantum-well structure it is shown that the electric fields arising from the charges of ionized dopants lead to strongly excitation dependent optical properties like significant differences between luminescence and gain wavelengths.

We present a comparison of experimental and microscopically based model results for optically pumped vertical external cavity surface emitting semiconductor lasers. The quantum well gain model is based on a quantitative ab-initio approach that allows calculation of a complex material susceptibility dependence on the wavelength, carrier density and lattice temperature. The gain model is coupled to the macroscopic thermal transport, spatially resolved in both the radial and longitudinal directions, with temperature and carrier density dependent pump absorption. The radial distribution of the refractive index and gain due to temperature variation are computed. Thermal managment issues, highlighted by the experimental data, are discussed. Experimental results indicate a critical dependence of the input power, at which thermal roll-over occurs, on the thermal resistance of the device. This requires minimization of the substrate thickness and optimization of the design and placement of the heatsink. Dependence of the model results on the radiative and non-radiative carrier recombination lifetimes and cavity losses are evaluated.

Modeling of high-power diodes poses several numerical problems. They require algorithms capable of capturing accurately the fast temporal and spatial dynamics in a broad spectral range. Another problem is how to reconcile vastly different time scales of various physical processes involved. We present an outline of a semiconductor laser simulation engine that incorporates both the first-principles many body gain calculations, and the carrier and heat transport simulation into an interactive computer laser model.

The material physics of digitally grown InAlGaAs quaternary alloy systems are investigated using Molecular Beam Epitaxy (MBE) grown layers. With MBE, arbitrary epitaxial alloy compositions can be achieved, without changing the group III elemental constituents flux rates, by simple sequential shuttering of the relevant fluxes. Monolayer fluctuations create inhomogeneities that lead to a broadening of the photoluminescence (PL) spectra. Multiple PL peaks are also seen in select alloy compositions.

Synchronization of chaotic semiconductor lasers has now been demonstrated experimentally using a variety of coupling schemes. Coupling methods include situations where the transmitter laser system is itself chaotic and drives a passive receiver system, both lasers are individually chaotic and, both lasers induce the chaos through mutual self-coupling. The qualitative dynamics for each of these scenarios is adequately captured by an appropriate set of coupled Lang-Kobayashi lumped rate equation models. Such lumped models however cannot distinguish between the possible coupling geometries realizable in real experimental systems and ignore multiple feedback from external reflecting surfaces. For example, real lasers may have AR/HR coated facets and there are several choices of placement of external reflectors and coupling paths relative to these facets. Moreover, nominally single mode FP lasers may exhibit pronounced multi-longitudinal mode dynamics in the presence of weak external reflection and DFB lasers may exhibit dual-wavelength operation or strongly asymmetric spatial hole-burning due to the presence of finite facet reflectivity.

A fully microscopic model is used to calculate the carrier capture times in quantum-well lasers operating at wavelengths in the 1.3 micrometers regime. The capture times are shown to be crucially dependent on the carrier confinement and therefore on the well and barrier materials. For a common well width of 6nm the capture times in InP/InGaAlAs and InP/InGaAsP structures are found to be in the 5ps range, whereas about a factor of ten longer times are predicted in GaInNAs/GaAs. By lowering the barriers using GaInNAs instead of pure GaAs or widening the well capture times similar to those in the InP-based structures can be obtained in the GaInNAs-based structures.

A general scheme for the determination of vital operating characteristics of semiconductor lasers from low intensity photo-luminescence spectra is outlined and demonstrated. A fully microscopic model for the calculation of optical properties is coupled to a drift diffusion model for the mesoscopic charge and electric field distributions to calculate photo-luminescence and gain spectra in barrier-doped semiconductor laser material. Analyzing experiments on an optically pumped multi quantum-well structure it is demonstrated that the electric fields arising from the space charges of ionized dopants contribute to strongly excitation dependent optical properties, such as significant shifts of the luminescence versus peak gain wavelengths.

Semiconductor and fiber amplifiers and lasers are amongst the most complex and critically important components in most modern optical telecommunications systems. The ever increasing demand for bandwidth places severe constraints on component design. Active materials need to be accurately characterized in terms of their optical properties. In addition, realistic simulation tools must be capable of resolving the multi-THz bandwidths while providing a rapid turn around to the system designer. We will report on the implementation of an extremely efficient algorithm running within an object-oriented simulation environment. As an illustration, we will present results showing how a WDM-based semiconductor optical amplifier and a TDM Mach-Zehnder interferometer gate can be optimized using rigorously computed and experimentally validated semiconductor optical material properties.

We calculate microscopically the gain and absorption, linewidth enhancement factor and carrier capture times for a GaInNAs/GaAs quantum-well laser operating in the 1.3 micrometers wavelength regime. The results are compared to those for an InGaAsP/InP and an InGaAlAs/InP structure with similar fundamental transition energies. The much higher confinement for carriers in the GaInNAs quantum well is shown to lead to larger gain bandwidths and, for low to moderate carrier densities, to lower linewidth enhancement factors than for the later two material systems. On the other hand, the high depth of the wells leads to longer carrier capture times in GaInNAs/GaAs.

High-power, femtosecond light filaments, also termed light strings, are experimentally observed to propagate over distances which substantially exceed the diffraction lengths that would correspond to their transverse dimensions. Thus, they provide a way to deliver high powers of focused light over long distance, and may potentially serve as light probes in remote sensing. We concentrate on a theoretical understanding of the underlying physics. In this talk, we review the results of our computer simulations providing insight into the rich spatio-temporal dynamics of this interesting phenomenon.

We outline physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers. The principal technical objective is to generate multi-watt CW or quasi-CW outputs with nearly diffraction limited beams, suitable for long distance free space transmission, focusing to small spots or coupling to single-mode optical fibers. The principal strategies are (1) optimization of facet damage thresholds, (2) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (3) design of laterally profiled propagation structures in lasers and amplifiers which suppress lateral reflections.

We are currently developing 2 semiconductor laser simulators built on a first-principles microscopic physics basis. The first is a PC-based, plane-wave simulator for both component and system-level design of low-power optoelectronic devices. The second is a supercomputer-based simulator that models the fully time-dependent and spatially-resolved optical, carrier, and temperature fields for arbitrary geometry, high-power semiconductor lasers. Both simulators are based on a comprehensive gain model that includes the relevant bandstructure of the quantum wells and confining barrier regions together with a fully quantum mechanical many-body calculation that takes all occupied bands into account.

Large-scale computer simulations of wide-beam, high-power femtosecond laser pulse propagation in air are presented. Our model, based on the nonlinear Schrodinger equation for the vector field, incorporates the main effects present in air, including diffraction, group-velocity dispersion, absorption and defocusing due to plasma, multiphoton absorption, nonlinear self-focusing and rotational stimulated Raman scattering. The field equation is coupled to a model that describes the plasma density evolution. Intense femtosecond pulses with powers significantly exceeding the critical power for self-focusing in air are simulated to study turbulence-induced filament formation, their mutual interaction via a low-intensity background, dynamics of the field polarization, and evolution of the polarization patterns along the propagation direction.

A robust, modular and comprehensive simulation model, built on a first-principles microscopic physics basis, includes the fully time-dependent and spatially resolved internal optical, carrier and temperature fields within an arbitrary geometry edge-emitting high-power semiconductor laser device. The simulator is designed to run interactively on a multi- processor shared memory graphical supercomputer by utilizing a highly efficient algorithm running in parallel over multiple CPUs. The experimentally validated semiconductor optical response is computed using a microscopic approach that includes the relevant bandstructure of the Quantum Well and confining barrier regions together with a fully quantum mechanical many-body calculation that takes all occupied bands into account. The latter quantity is introduced into the simulator via a multidimensional look-up table that captures the local dependence of the gain and refractive index of the structure over a broad range of frequencies and carrier densities. The simulator is designed in a modular form so as to be able to include differing device geometries (broad area, flared, multiple contacts, arrays, ..), filters (DBR or DFB grating sections), index/gain-guiding, temperature and current profiles and so on. Results will be presented for both broad area and MOPA devices.

Large-scale computer simulations of wide-beam, high-power femtosecond laser pulse propagation in air are presented. Our model, based on the nonlinear Schroedinger equation for the vector field, incorporates the main effects present in air, including diffraction, group-velocity dispersion, absorption and defocusing due to plasma, multiphoton absorption, nonlinear self-focusing and rotational stimulated Raman scattering. The field evolution is coupled to a model that describes the plasma density evolution. Intense femtosecond pulses with powers significantly exceeding the critical power for self-focusing in air are simulated to study turbulence-induced filament formation, their mutual interaction via a low-intensity background, dynamics of the field polarization, and evolution of the polarization patterns along the propagation direction.

A microscopic model for polarization switching in optically anisotropic VCSELs is presented. Our approach includes: (1) steady-state microscopic theory for the optical response of semiconductor quantum wells describing the dynamics of charge carriers and of interband polarizations in realistic bandstructures, including Coulomb-interaction correlations; (2) vectorial eigenmode calculation and the resulting expansion of the electromagnetic field in the laser in terms of vectorial eigenmodes of a whole structure, their polarization properties, mode-dependent losses and frequencies; (3) realistic model for optical anisotropies resulting from intentional or unintentional strain in an active quantum-well layer. The resulting steady-state input/output characteristics of linearly polarized microscopic eigenmodes of VCSELs are investigated in details. Linear stability analysis of these modes reveals the polarization switching behavior observed experimentally in practical VCSEL structures. We demonstrate that any nonzero uniaxial strain which may be present in the lattice structure (for instance, left over after the fabrication process) causes the laser to start lasing in a polarization eigenstate which is gain-preferred, but, for larger pumping currents, this polarization becomes unstable and the laser would switch to the orthogonal eigenstate.

A full scale simulation model, that resolves the spatio- temporal behavior of competing longitudinal mode and transverse filamentation instabilities in a wide variety of high brightness edge emitter geometries, is presented. The model is highly modular and is built on a first principles microscopic physics basis. The nonlinear optical response function of the semiconductor, computed for specific QW structures, covers the low-density absorption to high density gain saturation regimes. As an illustration of its robustness as a laser design tool, the model is applied to a monolithically integrated flared amplifier master oscillator power amplifier semiconductor laser.

Major many-body effects that are important for semiconductor laser modeling are summarized. We adopt a bottom-up approach to incorporate these many-body effects into a model for semiconductor lasers and amplifiers. The optical susceptibility function computed from the semiconductor Bloch equations (SBEs) is approximated by a single Lorentzian, or a superposition of a few Lorentzians in the frequency domain. Our approach leads to a set of effective Bloch equations. We compare this approach with the full microscopic SBEs for the case of pulse propagation. Good agreement between the two is obtained for pulse widths longer than tens of picoseconds.

Monolithically integrated flared amplifier master oscillator power amplifier (MFA-MOPA) semiconductor lasers are studied theoretically using a high resolution computational model which resolved times and longitudinal and transverse space dependencies and includes Lorentzian gain and dispersion spectra. The simulations show that, by altering the linear flare of the power amplifier into a nonlinear, trumpet- shaped flare, the dynamic stability range of the MOPA is increased by a factor of 3. This enables the MOPA to maintain a stable, nearly diffraction limited output beam for higher currents before the onset of transverse instabilities, large beam divergence and facet damage due to filamentation. Thus the MOPA will be able to emit an output beam of significantly higher power and brightness.

As part of a research program to understand and model eye damage produced by exposure to subnanosecond laser pulses, an effort is currently being made to model and analyze ultrashort pulse propagation from the cornea to the retina. Both analytical models and numerical simulations are being used to analyze the effects of self-focusing, laser-induced breakdown (LIB), and plasma-pulse interaction. The modeling effort is coupled with experimental measurements of LIB thresholds and plasma shielding for visible, picosecond (psec) and femtosecond (fsec) pulses in water, which serves as a reasonable simulant for the vitreous humor of the eye. Comparison of LIB thresholds to the critical power for self-focusing indicates that self-focusing has little effect on LIB thresholds for long psec pulses. For short psec and fsec pulses, however, numerical simulations show that self-focusing is critical to LIB in water. These results indicate that self-focusing may play a role in fsec pulse ocular damage, by influencing whether LIB and plasma-pulse interaction occur at the retina, in the vitreous, or both. Both the location of the LIB event and the amount of plasma shielding can significantly effect the degree of damage.

We report thermal effects revealed by a self-consistent treatment of plasma and lattice heating in vertical cavity surface-emitting lasers (VCSELs). The basic idea of our treatment is to couple the equations for carrier density and field amplitude in the conventional laser theory with those for two additional variables, the plasma and lattice temperatures. The CW operation of the VCSELs is investigated both for a fixed and for a self-consistently determined lattice temperature. In the first case plasma heating results in an increase of carrier density with pumping and thus in a pumping dependent frequency shift. In the latter case, both plasma and lattice heating induce a thermal switch-off of the laser as the pumping is increased. Furthermore, depending on the initial alignment of the cavity frequency and the ambient temperature of the device, heating can introduce a discontinuous threshold, exhibiting a bistability between lasing and nonlasing states. While some of our theoretical predictions are in qualitative agreement with known experiments, others await experimental verification.

The semiconductor Maxwell-Bloch equations provide a model that is grounded in the fundamental physics of semiconductors which include a variety of many body effects. Many of these effects are particularly noticeable when the semiconductor is probed with ultrashort pulses. We present computational results describing the computed behavior of model equations which describe the propagation of femto-second pulses in bulk GaAs. It is shown how the inclusion of additional physics modifies the predictions of the model. Among the effects that are discussed are plasma heating, plasma cooling, self-focusing, and memory effects.

Issues relating to modeling the full spatiotemporal dynamics of wide aperture semiconductor lasers and amplifiers are discussed. Included in the discussion are the limitations of the usual beam propagation approach, characteristics of the many-body light- semiconductor material interaction, spurious nonphysical instabilities which mimic numerical grid oscillations and novel subpicosecond pulse reshaping and compression effects. An explicit simulation is presented for a flared amplifier structure and the results are compared with those using a linear gain model.

A physical optics laser model based on a many-body semiclassical laser theory of the gain medium is used to investigate the gain medium effects on the modal stability of an unstable resonator semiconductor laser. Quantum confinement or strain are shown to result in single-mode operation over significantly wider ranges of unstable resonator configurations and gain medium excitation.

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Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews